1. Technical Field
The invention relates to a projection lens installed into a projection type display device and a projection type display device and, more particularly, a projection lens used in a projector system into which a light valve such as a transmission type liquid crystal, a reflection type liquid crystal or DMD (Digital Micromirror Device) is installed.
2. Description of the Related Art
A spreading speed of the recent projector is rapid, and the inexpensive projector is requested. Also, in respond to this request, the lens having a small number of constituent lenses is demanded of the projection lens used in the projector. However, performance deterioration is caused when the number of constituent lenses is reduced simply. In order to compensate such performance deterioration, the development of the lens having a small number of constituent lenses by using a lot of aspheric surfaces, and the like is proceeding. Such projection lens having a small number of constituent lenses and using the plastic aspheric surfaces is disclosed in JP 2003-287676 A, JP 2005-173494 A and Japanese Patent No. 3466002 (corresponding to U.S. Pat. No. 5,745,297).
However, since the lens disclosed in JP 2003-287676 A has a wide angle of view larger than 75 degree, such a lens is unsuitable for the application as the front projector that projects the image from the viewer side. In other words, when the image is projected within the screen at the above angle of view, the projector must be positioned near the screen and therefore the projector itself constitutes an obstacle to the user who watches the image on the screen. In order to prevent a situation that the projector itself acts as a visual obstacle, an image circle may be enlarged and the image may be shifted in the circle. In this case, since the lens should also be enlarged, a size of the device is increased.
Also, in JP2005-173494 A and Japanese Patent No. 3466002, the projection lens arranged mostly on the magnification side is formed of the plastic lens that does not endure the contact by the user's finger, or the like. Therefore, such projection lens is not suitable for the application as the front projector.